When measuring optical properties of reflective surfaces and determining the resulting spatially varying bidirectional reflectance function, one issue is to have improved measurement of gloss characteristics. Using only point light sources having fixed positions in an apparatus for irradiating a sample to determine the reflective properties of the sample may lead to a loss of spatial information for example in cases of anisotropic reflectance of the sample. Such a prior art approach is disclosed for example in Schwartz, C. et al., DOME II: A Parallelized BTF Acquisition System, in Proceedings of Eurographics Workshop on Material Appearance Modeling: Issues and Acquisition, Zaragoza, Spain, pages 25-31, Eurographics Association, June 2013. Another prior art approach is disclosed in U.S. Pat. No. 8,872,811 B1 which teaches to interpolate the positions of sensors of a measurement apparatus to make up for possible undersampling. This however involves some uncertainty in the correctness of the measurement data thus obtained and may lead to increased computational effort due to generation of additional data.
The invention therefore has the object of providing a method and apparatus capable of providing improved gloss measurement when determining a spatially varying bidirectional reflectance function for a specific structure while keeping the amount of data generated at a bearable level.
This object is reached by the subject-matter of any appended independent claim. Further features and aspects of the present invention are disclosed in the following and contained in the subject-matter of the dependent claims. Different features can be combined in accordance with the invention as long as technically sensible and feasible. For example, a feature of one embodiment which has the same or similar function of another feature of another embodiment can be exchanged. For example, a feature of one embodiment which supplements a further function to another embodiment can be added to the other embodiment.